#include <iostream>
#include <cstring>
using namespace std;

int main()
{
    char str[100];
    char c;
    int i = 0;
    while (cin >> c)
    {
        str[i++] = c;
    } // 填充字母字符串

    for (int j = 0; j < strlen(str); j++)
    {
        int count = 0;
        for (int k = 0; k < strlen(str); k++)
        {
            if (str[k] == str[j])
            {
                count++;
            }
        }
        if (count % 2 == 1)
        {
            cout << str[j] << endl;
            return 0;
        }
    }
    return 0;
}